An optical amplification device including a Semiconductor Optical Amplifier (SOA) has been put to practical use. An optical gain of the SOA is controlled by a driving current. That is, the SOA amplifies an input optical signal by an optical gain according to the driving current. Accordingly, in order to generate an optical signal with a desired optical level (e.g., a desired optical power), the optical amplification device detects the power of an input optical signal and applies the driving current according to the detected optical power to an SOA element.
However, the optical gain of the SOA depends on the temperature of the SOA element. Accordingly, the optical amplification device including the SOA has a function of controlling, for example, the surrounding temperature of the SOA element to be a constant temperature. In this case, the surrounding temperature of the SOA element is controlled using, for example a Peltier device.
When the surrounding temperature of the SOA is controlled to be a constant temperature using the Peltier device, a driving signal to be applied to the Peltier device is generated by, for example, a pulse width modulation (PWM) method. In a configuration where a driving signal is generated by the PWM method, power consumption for driving the Peltier device is suppressed. However, the PWM control is achieved by turning ON/OFF a switching device at a high speed. Also, in order to reduce the size of the optical amplification device, a PWM control circuit is mounted in the vicinity of the SOA element. Accordingly, when the driving signal of the Peltier device is generated by the PWM method, a switching noise is added to the optical signal amplified by the SOA.
The following are reference documents.    [Document 1] International Publication Pamphlet No. WO2006/048944    [Document 2] Japanese Laid-Open Patent Publication No. 2012-165127    [Document 3] Japanese Laid-Open Patent Publication No. 2013-229836    [Document 4] Shinsuke Tanaka et al., “Uncooled, Polarization-insensitive AlGaInAs MQW-SOA Module Operable up to 75° C. with Constant Current”, ECOC Technical Digest Tu. 6. LeSaleve2, 2011    [Document 5] Shinsuke Tanaka et al., “Compact, low-power-consumption AlGaInAs MQW-SOA modules”, General Conference of the Institute of Electronics, Information and Communication Engineers, C-4-25, 2012